It is known to use a process for expansion of bores and for interference mounting of attachment for increasing the fatigue life in an assembly of two metal parts. These processes for expansion and interference induce residual constraints of compression at the surface of the bore and locally in the part around the recess. These constraints have the effect of delaying the triggering and the propagation of cracks of fatigue in the immediate vicinity of the bore. The result is an increase in the fatigue life.
FIG. 1.A illustrates a known process called interference mounting for the assembly of two metal parts 1a, 1b. It consists in making a hole 4 in the two metal parts, the hole having a diameter Dhole that is less than the attachment diameter Dattachment that is the diameter of the attachment rod. The insertion of the attachment 3 in the attachment hole directly generates compression constraints at the periphery of the hole.
FIG. 1.B diagrammatically illustrates another example of a known process called an expansion process for generating residual compression constraints locally around the attachment hole 4 in an assembly of a metal part 1a with a second metal part 1b. An expansion tool 7 is used. The process comprises the following stages:                An attachment hole is made by means of a conventional drilling tool in the two metal parts, the diameter of the hole is selected so that it is adapted to the diameter of the expansion tool 7, i.e., the diameter of the hole Dhole is to be slightly less than the diameter of the expansion tool.        The expansion tool 7 that is called a burnisher is then passed through the hole that is made in the preceding stage; this tool comprises an olive-shaped expansion head 701 that has a diameter Dexpansion that is greater than the diameter of the attachment hole Dhole; its passage through the hole will exert a radial action on the inside wall of the hole, thus generating residual compression constraints in the two parts around the hole, and finally        The attachment hole is bored wider to adapt the diameter of the hole to the diameter of the attachment rod, and then the attachment is placed to maintain the assembly of the two parts.        
The two existing processes make it possible to generate constraints so as to increase the fatigue life at these working zones, critical due to the initiation of seams in loaded zones.
Within the framework of the assembly of a metal part and a composite part, it is no longer possible to apply processes as described above that would run the risk of greatly damaging the composite part.
The parts made of composite material exhibit exceptional properties in terms of resistance to mechanical fatigue and a strong rigidity while imparting a very low mass to the structures. These parts are implemented in particular in the aeronautics industry, including in greatly loaded structures. However, the assembly of these composite parts poses specific problems relative to the case of the metal parts.
Actually, the composite parts consist of structures that are obtained by stratification of resin-impregnated fibers, for example carbon fibers impregnated with an epoxy resin. Such a composite part has advantageous structural properties within the plane of fiber strata but is sensitive to delamination phenomena in a direction that is perpendicular to the planes, i.e., in the direction of the thickness of the parts, the direction used to place the attachment.
The compression forces exerted by the attachment means can produce the delamination phenomenon at the hole. In a general manner to prevent this delamination phenomenon, the constraints that appear at the attachment zone, i.e., at the interface between the inside wall of the attachment hole and the attachment element, should be reduced. For this purpose, and contrary to the case of an assembly of a metal part with another metal part, generally an attachment hole is made that has a diameter that is slightly larger than the diameter of the attachment so as to allow adequate play between the inner wall of the hole and the outside surface of the attachment to prevent interferences.
In aeronautical structures, the coexistence of the metal parts and composite parts leads to frequent assemblies of metal parts with composite parts. It may be a matter of junctions between two panels of different structures or local reinforcements, for example ribs, or metal stiffeners on a composite panel.
In such an assembly, either an assembly that comprises play between the wall of the holes and an attachment is selected, and the mounting is then unfavorable to the metal part in terms of fatigue life, or an assembly with interference is selected, and such a mounting runs the risk of damaging the composite part.
One solution would consist in independently making a hole for the attachment in the metal part and in the composite part, which are two separate parts, and then in generating residual compression constraints in the metal part in the absence of the composite part, and then, in a second step, placing the composite part against the metal part for the assembly. This solution is not satisfactory; actually in this case, it is necessary to predetermine precisely the positions of the holes so that they are aligned during the assembly for the passage of the attachment. This alignment cannot be produced industrially.
Another solution that is proposed in a prior, unpublished application consists in making an attachment hole in the metal part and in the composite part, and then in making a new bore in the hole beside the composite part so as to obtain a larger inside diameter than the diameter of the expansion tool. Thereby, the existence of two inside diameters in the attachment hole makes it possible to generate only residual compression constraints in the metal part.
However, this solution dictates having an access on the side of the composite part and even an access on each side.